In today's security systems it is desirable for the central control station to be able to communicate with a security system user during an alarm situation. The operator at the central control station can ask the user for the security code and the type of assistance the user requires. The user can respond with the required information. In order to accomplish this in a wireless security system, the operator's voice, which enters the security system through the phone line connected to the panel, must be transmitted to the remote devices of the security system such as the keypad, the pendant, etc. The security system panel accomplishes this by enabling a base station connected to the panel to seize the phone line. The base station converts the audio signal from the phone line into an FM signal and transmits the FM signal to the remote security system devices as well known in the art. The remote security system devices demodulate the FM signal and send the recovered audio signal through a speaker to the user. When the user communicates with the operator, the opposite operation takes place with the remote devices modulating the audio signal and the base station demodulating the FM signal to an audio signal. The audio signal is then sent to the operator through the phone line.
The modulation of the audio signals and demodulation of the FM signals, techniques well known in the art, introduce noise and in some situations drop outs into the audio signal. This occurs because the circuits that perform the modulation and demodulation use discrete components that are manufactured within certain tolerances. The tighter the tolerances are, the more expensive the components are. In order to be cost effective, the devices are typically manufactured with less expensive components. In addition, operating characteristics of these components may change with age and with temperature variations. These component variations may cause a 90 KHz FM carrier (or base) frequency to become lower than 80 KHz or higher than 100 KHz. When the audio signal, which has a 20 KHz bandwidth is imposed on the carrier frequency the resulting FM signal could be in the range of 50 KHz to 70 KHz, in the range of 110 KHz to 130 KHz, or somewhere in between. When the demodulation circuit tries to decode the audio signal from the FM signal that has a carrier frequency that has drifted significantly, the resulting audio signal becomes distorted. One way to overcome this problem is to increase the dynamic range of the demodulation circuit by increasing its bandwidth and decreasing its sensitivity to cover the entire 50 KHz to 130 KHz range, but this allows more noise into the decoded signal. Another solution is to trim, or adjust, each security device during installation to make sure that the audio signal is not distorted. This solution is costly to the installers of the security device. It is desirable to install multiple devices that are low cost and that communicate clearly right off the shelf.
It is therefore an object of the present invention to provide an alarm device that can demodulate a wide dynamic range of FM signals.
It is a further object of the present invention to provide an alarm device that generates an accurate audio signal.
It is a further object of the present invention to provide an alarm device that self-adjusts for accurate FM communication.
Finally it is a further object of the present invention to provide an alarm system that is inexpensive to manufacture and install.